Whether or not the US is directly party to an Israeli attack on Iran's nuclear facilities – if it happens – American officials say they expect Tehran to blame Washington and to retaliate by hitting US targets.

But officials in Washington are calculating that Iran would not seriously escalate the conflict.

"Iran can't go force on force against us," one US official told me. "It might sink a ship or two. It might hit us in Afghanistan. But we can absorb the hits."The Americans and Israelis consider it a virtual certainty that Iran would respond to any attack by launching missiles against Israel or with some other strike at the Jewish state. But US officials consider it less certain that Tehran would do anything that would draw the Americans into an extended conflict.

Last month, the director of the US Defence Intelligence Agency, General Ron Burgess, told the Senate armed services committee: "Iran can close the Straits of Hormuz, at least temporarily and may launch missiles against United States forces and our allies in the region if it is attacked. Iran could also attempt to employ terrorists surrogates worldwide. However, the agency assesses Iran is unlikely to initiate or intentionally provoke a conflict."

US officials expect what they describe as a calibrated response from Tehran, and point to the example of Iran's role in the insurgency against American forces occupying Iraq.

The Pentagon accused Iran of supplying Shiite militias with explosives, rocket propelled grenades and Katyusha rockets for attacks on American soldiers. The state department responded by designating Iran's revolutionary guard and Quds Force as terrorist organisations for their support of Iraqi insurgents. But both the US and Iran avoided open, direct conflict.

Similarly, US officials think that the Iranians may hit back against American troops in Afghanistan via the insurgents there. They may also target oil wells and seek to stir trouble among the US's Arab allies.